In conventional molding practice, alignment of the flask to the pattern, and flask-to-flask, is achieved by means of pins and bushings. More specifically, in most instances pins are provided on the pattern which can engage bushings on the drag flask at the mold making station. These same bushings are then used to engage pins on the cope flask at the closing station, where the drag and cope flasks are assembled. The above achieves the alignment requirement for patterns to flask, and flask-to-flask.
One of the problems frequently encountered in conventional molding systems is a casting flaw attributable to parting line shift. One of the principal causes of this casting defect is the general problem associated with the clearances required between pins and bushings, and the increase in those clearances caused by wear on these critical alignment parts. Typically, clearances on the order of ten thousandths of an inch are required between pattern pins and their mating bushings on the drag in order to prevent binding action as the flask is lowered onto the pattern or later on when it is drawn. Similar clearances are required in making the cope, and the same clearances are also then encountered in the match between the cope and the drag.
In addition to these initial clearances, it is not uncommon to see wear on pins and/or bushings which can often accumulate to an additional ten thousandths of an inch. It can therefore be seen that with worn pins and bushings, a drag might be shifted relative to its pattern as much as fifteen thousandths of an inch from a theoretical true desired position. Similarly, a cope can be shifted by a corresponding amount, but in the opposite direction. When the cope and drag are eventually closed, an error of as much as forty-five thousandths of an inch can occur between the cope and drag parting surfaces from these variables alone. Such a shift in mating surfaces can result in minimum wall thickness in the resultant casting not being met. To compensate for such a mating shift, it is necessary in pin and bushing castings to enlarge overall the pattern so that wall-thickness specifications will be achieved. This means pouring more metal than necessary for each casting.
A recent development in the art of casting which eliminates many of the alignment problems experienced in pin and bushing casting is presented in commonly assigned U.S. Pat. No. 4,628,986, issued Dec. 16, 1986, to Donald L. Southam. As disclosed therein, a molding register system is provided which eliminates nearly all the error caused by pin and bushing clearances and wear. Registry is achieved by urging the rectangular flask to two fixed surfaces (or one long surface) on the long axis and one fixed surface on the shorter axis of the flask. This registry principle is applied in both the cope and drag mold making locations, and also the closer location, where the mold halves are assembled, and utilizes the same flask surfaces for all registration purposes.
In implementing this registry scheme, it is critically important to assure parallelism between the cope and drag parting surfaces during the closing operation. Failure to achieve parallelism can lead to errors in the closing operation resulting in objectionable core shaving.